The absorption band arising from the tranfer of an electron from a reducing site to an oxidizing site in the same molecule (intervalence absorption) is a convenient and powerful means of probing paths for spontaneous electron transfer. The weak interaction between the redox centers seem particularly pertinent to electron transfer in biological systems. Work on intervalence absorption for molecules containing pentaammineruthenium(II) and (III) at fixed known distances will be continued, emphasizing molecules which contain two thio-ether groups as binding sites for the metal ions. The complexes of Ru(II) and Ru(III) with pyrimidine and purine are being studied. Of special interest are the spectra and the comparison of the different polar groups available on purine as bridging sites for metal ions. The properties of (Ru(NH3)5imC)3ion, 2ion where imC represents carbon-bound imidazole, and of OsC6H6 2ion aq. are being investigated, with the particular goal of establishing their affinities for the kind of nucleophilic centers that appear in proteins and polynucleotides.